Follicular dendritic cell sarcoma (FDCS) of the duodenum is extremely rare. We describe the contrast-enhanced CT and FDG PET/CT findings in a case of duodenal FDCS. Contrast-enhanced CT revealed a large, irregular mass originating from the horizontal portion of the duodenum. The mass demonstrated heterogeneous enhancement, with internal areas of low-attenuation necrosis and coarse calcifications. The mass showed significantly increased FDG uptake. The patient underwent surgical resection, and histopathologic examination revealed a spindle cell tumor positive for CD21 and CD35, confirming the diagnosis of FDCS.

Renal schwannomas are exceedingly rare benign tumors originating from Schwann cells, with fewer than 50 cases reported in the literature. The "ancient" variant refers to long-standing schwannomas that develop degenerative changes, which may mimic the aggressive radiological features of renal cell carcinoma.This diagnostic overlap often leads to preoperative misdiagnosis and can significantly impact surgical decision-making, often resulting in radical rather than nephron-sparing approaches. A 48-year-old White woman presented with a 3-month history of intermittent left flank discomfort.Imaging demonstrated a 6-cm complex cystic mass at the upper pole and hilum of the left kidney, characterized by thickened, enhancing septations (up to 7.5mm) and a 13-mm mural nodule, consistent with a Bosniak IV cystic renal mass.No metastases were detected.Owing to the high suspicion of cystic renal cell carcinoma and the complex hilar location, she underwent laparoscopic left radical nephrectomy.Histopathological evaluation revealed a spindle cell neoplasm with Antoni A and B patterns and extensive degenerative changes, including hemorrhage and calcification, confirming a diagnosis of ancient schwannoma.Immunohistochemistry showed strong, diffuse S-100 positivity and SOX10 expression.At 12-month follow-up, the patient remained asymptomatic and disease-free. Ancient renal schwannomas pose substantial diagnostic challenges as they frequently masquerade as renal malignancies on imaging.While definitive diagnosis currently relies on histopathological analysis, maintaining awareness of this rare entity is important when evaluating complex cystic renal masses.Greater diagnostic awareness may help inform surgical planning and, in selected cases, allow consideration of nephron-sparing approaches.

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High-grade Spindle Cell Sarcomas (SCS) are rare, aggressive soft-tissue tumours that can be extremely challenging to diagnose, particularly when recurrent or when they morphologically overlap with other spindle-cell neoplasms. This case involves a 76- year-old Indian female who presented with a recurrent right posterolateral thigh swelling six months after surgical excision of a poorly differentiated liposarcoma. The swelling was firm, non tender, and subcutaneous, with no associated systemic symptoms. Primary Fine-Needle Aspiration Cytology (FNAC) revealed polygonal and spindle-shaped cells with hyperchromatic nuclei, mild pleomorphism, and prominent nucleoli, leading to an impression suggestive of malignant melanoma. Ultrasonography (USG) showed a heterogeneous hypoechoic lesion, while Magnetic Resonance Imaging (MRI) demonstrated a 3.5×2.6×1.6 cm subcutaneous mass with mild septal thickening, scarring, and postcontrast enhancement—findings suggestive of recurrent liposarcoma. Intraoperative frozen-section analysis shifted the differential diagnosis to Dermatofibrosarcoma Protuberans (DFSP) versus liposarcoma due to the spindle-cell morphology. Gross examination of the total specimen (measuring 6.5×5×4 cm) revealed a 2.8×1.8×1.5 cm protuberant, greyish-white, firm, lobulated nodular mass arising from the skin. Histopathological evaluation showed a storiform and fascicular pattern of spindle-cell proliferation infiltrating the dermis and subcutis, with pleomorphism and frequent mitoses, confirming a diagnosis of high-grade SCS. This case highlights the limitations of FNAC and frozen-section analysis in the evaluation of recurrent soft-tissue tumours and underscores the importance of integrating histopathology, imaging findings, and prior clinical history for accurate diagnosis.

ALK Rearrangements in Cutaneous Tumors: Molecular Insights and Emerging Entities.

The 5th edition of the World Health Organization (WHO) classification of skin tumors introduces a dedicated chapter on cutaneous soft tissue tumors, providing a comprehensive, standardized reference with updated diagnostic criteria that directly inform routine dermatopathology practice and molecular diagnostics. This edition incorporates several key changes, including newly recognized entities such as EWSR1::SMAD3-rearranged fibroblastic tumor, neurotrophic tyrosine receptor kinase (NTRK)-rearranged spindle cell neoplasm, superficial CD34-positive fibroblastic tumor, and CRTC1::TRIM11 cutaneous tumor. Diagnostic terminology has also been refined; for example, the term 'atypical intradermal smooth muscle neoplasm' replaces 'cutaneous leiomyosarcoma' for lesions confined to the dermis, whereas the designation leiomyosarcoma is reserved for tumors with overt subcutaneous infiltration. In addition, epithelioid fibrous histiocytoma has been reassigned to the family of tumors of uncertain differentiation. This review summarizes the key updates and newly recognized entities in the chapter on cutaneous soft tissue tumors in the 5th edition of the WHO classification of skin tumors, emphasizing their clinicopathological and molecular implications.

ABSTRACTTRAF7 mutations are a rare occurrence in human cancer and have recently been described in a group of mesenchymal tumors with varying clinical course. Herein, we expand the spectrum of TRAF7‐mutated fibromyxoid spindle cell tumors by reporting the first case to arise in bone. A 60‐year‐old woman presented with right knee pain and was incidentally found to have a left distal femur lesion, which was first detected 20 years prior when it was favored to be benign. Recent imaging studies revealed significant interval growth with focal cortical destruction and soft tissue extension. Histologic examination showed a bland spindle cell neoplasm with fibrous to myxoid stroma. Rare mitotic figures were present; necrosis and marked cytologic atypia were absent. Immunohistochemical work‐up showed that the spindle cells only demonstrated focal cytoplasmic staining with L1CAM, and whole exome sequencing identified a TRAF7 p.Y563C missense mutation. The tumor was resected, and the patient is recovering well at 2 months with no evidence of local recurrence or distant disease. This report is the first known case of a TRAF7‐mutated fibromyxoid spindle cell tumor of bone with the longest clinical follow‐up reported to date.

Plaque-like CD34-positive dermal fibroma (PDF), previously termed medallion-like dermal dendrocyte hamartoma, is a rare CD34-positive superficial spindle cell fibroblastic tumour that may closely mimic dermatofibrosarcoma protuberans. Recent studies have identified recurrent kinase gene fusions in a subset of congenital and paediatric CD34-positive plaque-like superficial spindle cell tumours with overlapping clinicopathologic features, highlighting an emerging molecular framework for this group. However, the molecular spectrum of lesions meeting classic histopathologic criteria for PDF remains an area of active investigation. Four cases of PDF were identified retrospectively, and histopathologic and immunohistochemical findings were reviewed. RNA-based next-generation sequencing was performed to evaluate for potential oncogenic gene fusions. All cases demonstrated a superficial, dermal-based proliferation of bland spindle cells with a preserved Grenz zone and diffuse CD34 expression, without S100 expression. Two of four cases harboured in-frame NTRK3 fusions, including a novel SPTBN1::NTRK3 fusion and a PPFIBP1::NTRK3 fusion, both retaining the NTRK3 tyrosine kinase domain. Available clinical follow-up demonstrated indolent behaviour in all patients. NTRK3 gene fusions occur in a subset of plaque-like CD34-positive dermal fibromas, providing additional molecular insight into this rare superficial spindle cell lesion. Together with emerging literature, these findings support a role for recurrent kinase gene rearrangements in a subset of CD34-positive plaque-like superficial spindle cell tumours, while underscoring the continued importance of clinicopathologic correlation in defining this evolving spectrum.

Myxofibrosarcoma (MFS) is a rare malignant soft tissue sarcoma, typically affecting elderly patients and commonly arising from the extremities. Its occurrence in older individuals with atypical locations, such as retroauricular, is uncommon. A 50-year-old female presented with a history of recurrent left huge retroauricular mass, a painless and hard mass, not mobile, not tender, attached to the skin, approximately 4*3 cm. Initial imaging suggested a benign lesion; however, histopathological results of the excised mass revealed a spindle cell neoplasm with a lot of myxoid changes that indicate High-grade Myxofibrosarcoma. Immunohistochemistry confirmed a diagnosis of high-grade MFS. The patient underwent surgical excision of the retroauricular mass with clear margins and remains under regular follow-up with no signs of recurrence to date. In this case, the diagnostic tools of MFS are mainly used in elderly patients with atypical anatomical locations. The imaging is a nonspecific tool to confirm diagnosis, and the histopathological examination is an accurate diagnostic tool for MFS. The mainstay of MFS treatment is surgical excision, and in this case, the patient received Adjuvant Radiotherapy and systemic chemotherapy therapy (doxorubicin and ifosfamide) due to high-grade or multiple recurrences, adjuvant therapy and Regular follow- up; are essential to prevent local recurrence. This case contributes to the limited data on MFS in sub-Saharan Africa, to increase awareness and reporting, and to better understand epidemiology and management strategies in this region.

Pediatric adrenal tumors are uncommon, with the most common being peripheral neuroblastic tumors. Other neoplasms like adrenal cortical tumors, pheochromocytoma, and myelolipoma may also be encountered in this age group. Rare instances of mesenchymal tumors such as Ewing sarcoma and rhabdomyosarcoma have been described. We report the fine needle aspiration cytology (FNAC) findings of the first case of BCOR-altered sarcoma occurring in the adrenal gland. A 13-year-old boy presented with severe abdominal pain. Imaging revealed a mass in the right adrenal gland. FNAC showed loosely cohesive clusters and singly dispersed medium-sized polygonal to spindled tumor cells with scant to moderate delicate cytoplasm and normochromic, mildly pleomorphic nuclei with fine granular chromatin. Slender fibrovascular cores traversed tumor fragments. Histopathology of the adrenalectomy showed sheets of round to spindled tumor cells interrupted by thin fibrovascular septae, with foci of necrosis. BCOR, SATB2, and cyclin D1 were positive, while CD99, NKX2.2, WT1, desmin, and myogenin were negative. Fluorescence insitu hybridization revealed BCOR::CCNB3 fusion, consistent with BCOR-rearranged sarcoma. BCOR-rearranged sarcomas are rare malignant neoplasms. As primary treatment is chemotherapy, accurate diagnosis on limited material such as FNAC is critical. The recognition of cytological features and follow up with appropriate ancillary testing has the potential to improve management of these patients.

The diagnosis of primary renal synovial sarcoma (PRSS) is challenging due to nonspecific clinical semiology mimicking renal carcinoma and significant histologic overlap with other kidney tumors like sarcomatoid renal cell carcinoma, clear cell sarcoma of the kidney (CCSK), and BCOR-altered sarcoma, necessitating molecular confirmation via the detection of SS18::SSX gene fusion. This case report highlights a 39-year-old male patient presenting with abdominal pain and hematuria. Imaging revealed a large, heterogeneously enhancing left renal mass. Left-sided nephrectomy revealed a tumor, which on histopathology revealed a monophasic spindle cell neoplasm with areas of necrosis. Immunohistochemistry (IHC) demonstrated diffuse positivity for pan-keratin (clone AE1/AE3), EMA, CD99, BCL2, cyclin D1, SS18, and paradoxically, BCOR-a marker classically associated with BCOR-rearranged sarcoma/CCSK. Fluorescence in situ hybridization assay confirmed SS18 gene rearrangement without the presence of BCOR gene molecular alterations, establishing the diagnosis of monophasic synovial sarcoma. This case report highlights a critical diagnostic pitfall comprising of aberrant BCOR immunoreactivity, usually reported in synovial sarcomas of soft tissues, but hitherto unreported in PRSS. This aberrant immunoexpression of BCOR probably reflects epigenetic dysregulation, occurring in the absence of underlying BCOR genetic rearrangements. Coexpression of BCOR and cyclin D1 can also lead to a misdiagnosis as CCSK. Therefore, owing to overlapping IHC profiles, definitive diagnosis of PRSS warrants SS18::SSX gene molecular testing to avoid misclassification, especially given the therapeutic and prognostic implications. Integration of morphology, IHC, and molecular studies remains paramount for accurate diagnosis of such rare renal spindle cell neoplasms.

Spindle cell mesenchymal neoplasms comprise a heterogeneous group of tumors, with recent molecular advances uncovering novel pathogenic mechanisms. We report the case of a 22-year-old woman presenting with progressive dyspnea. A computed tomography (CT) scan revealed a solid mass located in the right cardiophrenic angle. Histological examination showed a spindle cell neoplasm composed of medium-sized, elongated cells with scant eosinophilic cytoplasm, arranged in storiform and whorled patterns. Immunohistochemical staining was negative for lineage-specific markers. Fluorescence in situ hybridization (FISH) detected an EWSR1 gene rearrangement in 15% of the neoplastic cells, and RNA sequencing demonstrated a novel EWSR1::HOXB8 fusion. This case highlights the relevance of EWSR1-driven fusions in spindle cell mesenchymal neoplasms and emphasizes the need for further exploration into their clinical and biological behavior. After 5 months of chemotherapy, the patient showed moderate clinical improvement.

Solitary fibrous tumor (SFT) is a rare fibroblastic neoplasm that may occur at various anatomic sites, whereas vulvar involvement is exceptional. The myxoid variant of SFT, defined by extensive myxoid stromal change, is exceedingly uncommon and may present diagnostic challenges due to atypical radiologic and histologic features. We report a case of a 35-yr-old woman with a slowly enlarging vulvar mass radiologically interpreted as a benign cystic lesion. Histologic examination revealed a predominantly myxoid spindle cell tumor with staghorn vascular features in its periphery. Immunohistochemistry showed diffuse nuclear STAT6 and CD34 positivity, and molecular analysis confirmed a NAB2:::STAT6 fusion. This case highlights the diagnostic pitfalls associated with myxoid change in vulvar mesenchymal tumors and underscores the importance of immunohistochemical and molecular confirmation.

Abstract DFSP is a rare, low-grade soft tissue sarcoma characterized by locally aggressive growth and low metastatic potential. The fibrosarcomatous (FS) variant, which occurs in approximately 10-15% of cases, is associated with increased biological aggressiveness, higher rates of local recurrence, and a greater risk of distant metastasis, particularly to the lungs. DFSP involving the breast is exceptionally rare, especially in male patients, making diagnosis and treatment more challenging due to its clinical similarity to breast carcinomas. A 65-year-old male, ex-smoker, presented with a progressively enlarging left breast mass over a four-month period. On physical examination, the lesion appeared as an irregular, indurated, mobile mass, measuring 7x6 cm, with hyperemia and cutaneous infiltration. Breast ultrasonography demonstrated a solid, hypoechoic, heterogeneous, lobulated nodule (BI-RADS 4C). Chest CT revealed a 7.1x5.7x4.5 cm mass with no clear cleavage plane with the pectoralis major muscle. Core biopsy demonstrated a spindle cell neoplasm, CD34-positive and Factor XIIIa-negative, consistent with DFSP. Clinical staging was cT4b N0 M0. The patient underwent a wide local excision via simple mastectomy with partial resection of the pectoralis major muscle to achieve adequate deep margins, and immediate reconstruction was performed using an abdominal fasciocutaneous flap. Histopathological analysis confirmed DFSP with FS transformation (DFSP-FS), high mitotic rate, and all surgical margins were negative, with the closest deep margin exceeding 0.5 cm. Immunohistochemistry (IHC) showed diffuse CD34 positivity, negative S-100, p63 and CK, and Ki67 >30% in fibrosarcomatous areas. DFSP involving the breast is extremely rare, particularly in males, with few reported cases. It contributes to diagnostic delays and frequent misclassification as more common breast malignancies. Clinical presentation can be insidious. Imaging exams, although useful for anatomical delimitation, are not specific for diagnosis. Ultrasonography and mammography may suggest solid lesions, but magnetic resonance imaging is superior in assessing the extent of local invasion and involvement of adjacent structures. Definitive diagnosis relies on histopathology and IHC, with CD34 positivity and absence of epithelial markers. The presence of fibrosarcomatous areas, with high mitotic activity and an elevated Ki67, indicates aggressive transformation and a worse prognosis. Wide surgical excision with negative margins remains the cornerstone of treatment to minimize recurrence. Simple mastectomy with wide margins, including resection of adjacent muscle tissue when necessary, is recommended in cases of extensive involvement or fibrosarcomatous transformation, as in the present report. Adjuvant radiotherapy is recommended in situations of positive margins, unresectable disease, or locoregional recurrence, contributing to the reduction of the local recurrence rate, as evidenced by retrospective studies and systematic reviews. Radiotherapy should also be considered in cases of DFSP-FS, due to its potential aggressiveness. The use of imatinib, a tyrosine kinase inhibitor that acts on the characteristic COL1A1-PDGFB gene fusion of DFSP, has proven effective in advanced, unresectable, or metastatic cases, offering an important therapeutic option for patients with refractory disease. This rare case of DFSP-FS infiltrating the male breast underscores the diagnostic challenges and therapeutic considerations associated with this entity. It highlights the critical importance of a multidisciplinary approach for timely diagnosis and optimal surgical management, ultimately improving patient outcomes. Citation Format: G. Neri Ferreira, L. Novis Leite Pinto, L. Martins de Brito Moraes, D. Franco Vieira de Oliveira, A. Coelho de Oliveira, A. Pereira Correia Carneiro, A. Dominique Nascimento Lima. Dermatofibrosarcoma protuberans (DFSP) with fibrosarcomatous transformation involving the male breast: A rare case report and comprehensive review of clinicopathologic features [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS5-06-15.

A BSTRACT Primary treatment for soft tissue leiomyosarcoma (LMS) is surgical excision; however, its frequent recurrence necessitates the options such as chemotherapy, radiation, and advancements like MR-guided radiation therapy (MRgRT) improves treatment outcomes. A 59-year old postmenopausal female presented with complaints of lower abdominal pain and lower backache for the past 6 months. A positron emission tomography and computed tomography (PET CT) whole body revealed an enlarged right paracaval lymph node. Laparoscopic excision was carried out. Intraoperatively, the mass was noted in the retrocaval region, adherent and arising from the posterior wall of the inferior vena cava (IVC). Histopathological examination revealed a malignant spindle cell tumor likely LMS. While immunohistochemistry showed positive for H-caldesmon and smooth muscle actin and negative for estrogen receptor. Postoperative magnetic resonance imaging (MRI) revealed focal, irregular soft tissue along the posterior wall of the IVC. She was planned for adjuvant radiation using MRgRT, and generated plan of the day was delivered under continuous real-time MRI of the target. On 3-monthly follow-up, PET CT scan revealed complete resolution of metabolic activity and a significant decrease in the size of the lesion.

Atypical fibroxanthoma (AFX) and pleomorphic dermal sarcoma (PDS) are cutaneous neoplasms that fall along a spectrum. PDS is more aggressive than AFX with higher rates of local and distant metastases. Diagnostic biomarkers for AFX and PDS are lacking and therefore these tumors are diagnosed only after excluding other dermal spindle cell neoplasms, including cutaneous leiomyosarcoma (cLMS), spindle cell melanoma (SCM), and sarcomatoid squamous cell carcinoma (sSCC). To identify clinically valuable biomarkers, we contrast the tumors within the diagnostic differential using single-cell RNA sequencing and bulk proteomic data. Gene Ontology (GO) analysis of transcripts and proteins enriched in AFX/PDS identified multiple shared pathways associated with cell adherence and the extracellular matrix. We identify that LRP1, LTBP2, and NAV1 are all enriched in AFX/PDS over other tumors in the differential at both the level of mRNA and protein. IHC reveals that LRP1 is 90% sensitive and 73% specific for AFX/PDS in a cohort of AFX, PDS, cLMS, SCM, and sSCC. This outperforms published data for CD10, which is currently used clinically (sensitivity 83.5% and specificity 50%). When used in conjunction with LTBP2, specificity for AFX/PDS within the differential rises from 73% to 93%. These findings suggest that LRP1, particularly if evaluated in conjunction with existing stains, can improve diagnostic accuracy for AFX and PDS.

SMARCA2::CREM fusions have been reported in a small number of rare and heterogeneous neoplasms across various anatomic sites. Although several morphologic features overlap among the reported cases, they can pose significant diagnostic challenges due to their variable morphology and nonspecific immunophenotype. They may also be underrecognized in clinical practice because SMARCA2 and CREM are not routinely included in many clinical fusion gene panels. SMARCA2::CREM fusion genes have been most frequently found in intracranial mesenchymal tumors and hyalinizing clear cell carcinomas, and previous studies of SMARCA2::CREM fusion-positive tumors have consistently shown the retained expression of SWItch/Sucrose Nonfermentable (SWI/SNF) complex proteins, including BRG1, BRM, and INI1. In this case report, we describe the first gynecologic tumor harboring the SMARCA2::CREM fusion gene, diagnosed in a 39-year-old woman. Histologically, the tumor exhibited spindle cell and epithelioid components, with rhabdoid features in some areas. Immunohistochemistry demonstrated epithelial marker positivity in the epithelial component, along with retained expression of BRG1 and INI1. Whole-transcriptome sequencing revealed SMARCA2::CREM fusion that was not detected by the in-house fusion gene panel. This case contributes additional data to the limited literature on tumors with SMARCA2::CREM fusions. It highlights the potential utility of comprehensive molecular testing, including broader fusion panels or whole-transcriptome sequencing, in the evaluation of diagnostically challenging uterine neoplasms.

Epstein-Barr virus (EBV)-associated smooth muscle tumor (EBV-SMT) is a rare spindle cell tumor found in immunocompromised patients with HIV. This case report presents a patient with HIV-positive Epstein-Barr virus (EBV)-associated smooth muscle tumor (EBV-SMT) involving the brain and spine-a 39-year-old Hispanic female with HIV presented with severe headache, neck pain, and seizure-like activity. Magnetic resonance imaging (MRI) revealed multiple brain masses, prompting a diagnostic cerebral angiogram and subsequent surgery to remove them. Despite initial improvement, the patient later succumbed to aspiration pneumonia and cardiac arrest. Epstein-Barr virus (EBV)-associated smooth muscle rare tumors (EBV-SMTs) in HIV patients with low CD4 counts require clinical suspicion and Epstein-Barr virus (EBV) involvement in brain tumor diagnosis. Early HIV detection, CD4 monitoring, and antiretroviral therapy (ART) management can decrease the risk of these malignancies.

Inflammatory myofibroblastic tumors of the skin and mucosal sites: A clinicopathological and molecular analysis of 3 cases with emphasis on differential diagnosis.

Low-grade fibromyxoid sarcoma (LGFMS) is a deceptively bland spindle cell neoplasm with malignant potential, most commonly arising in the extremities and trunk, and limited to at least110 cases in the head and neck. This report describes the first molecularly confirmed LGFMS of the floor of the mouth demonstrating a FUS::CREB3L2 gene fusion. An 18-year-old male presented with a painless floor of mouth mass of 1-year duration that was clinically suspected to represent a ranula. The mass was subjected to excisional biopsy. Histological examination was salient for an infiltrating proliferation of bland spindle cells arranged in whorled patterns within a collagenous stroma with focal myxoid change. Immunohistochemical analysis revealed diffuse MUC4 expression, narrowing the diagnosis. Targeted RNA sequencing identified a FUS::CREB3L2 fusion involving FUS exon 6 and CREB3L2 exon 5, confirming LGFMS. At 1-year follow-up, the patient showed no clinical evidence of disease and no detectable circulating tumor DNA. This case highlights the critical role of molecular testing in the diagnosis of LGFMS and emphasizes the importance of appropriate management strategies, including aggressive local control and long-term surveillance, given the tumor's risk of late recurrence and distant metastasis.